Electrolytic preparation of iron powder



United States Patent @flise Patented June 26, 1962 3,041,253 ELECTROLYTIC PREPARATION OF IRON PUWDER Rene Audubert, Bievre, and Henri de Lacheisserie, Versailles, France, assignors t Societe dElectro-Chimie dElectro-Metallurgie et des Acieries Electriques dUgine, Paris, France, a corporation of France No Drawing. Filed Feb. 5, 1957, Ser. No. 638,223 Claims priority, application France Feb. 8, 1956 5 Claims. (Cl. 204-) This invention relates to the electrolytic preparation of iron powder with determined magnetic characteristics, in particular, a high coercive force.

It is known that the finer the size of an iron powder is, the higher is its coercive force. The preparation of very fine iron powder by electrolytic means has been attempted often but the products obtained until now were either too coarse or too oxidized to produce regularly high magnetic characteristics.

Applicants have found that it is possible to obtain iron powders of diverse grain size and having well determined and various properties in accordance with the grain size by electrolyzing ferrous salts under well determined conditions as set forth hereinafter.

The present process consists in electrolyzing a dilute solution of ferrous salt carefully freed from oxygen and cut off from all contact with oxygen, at a current density which is higher as the concentration of the electrolysis solution is higher, in periodically detaching from the cathode the powder fonned thereon and in separating the powder from the bath in such manner that it has no contact with oxygen.

The electrolysis solution may be an extremely pure solution of ferrous salt or it may contain small quantities of ferric salts.

Although the ferrous salt concentration may vary considerably, it strongly affects the dimensions of the obtained grains. All other conditions being the same, decreasing the concentration of the ferrous salt increases the fineness of the grains. Although it is possible to use other concentrations, those between 0.05 and 0.5 mol per liter are particularly suitable.

The pH value of the electrolysis solution is extremely important. If the pH is too low, a noticeable part of the current is consumed uselessly by hydrogen discharge. If the pH is too high, a hydroxide precipitate is formed in the solution which mixes with the iron powder and, if abundant, detracts from some of its desirable characteristics, in particular, its specific magnetization. It is known that for each concentration of a ferrous salt solution there is a pH at which the precipitation of the corresponding hydroxide begins. The pH at which ferric hydroxide coming from a ferric salt solution begins is, for equal concentrations, lower than that of the ferrous hydroxide coming from a solution of the corresponding ferrous salt. It is also known that, for a given salt, the pH, at which precipitation begins, increases as the con centration of the bath decreases. In a ferrous salt solution containing only a little ferric salt, it is possible to have a precipitation of ferrous hydroxide before precipitation of ferric hydroxide begins. In accordance with the present invention, the pH of the solution is chosen such that it is slightly lower than the value corresponding to the appearance of the first hydroxide precipitate.

If one were to use a ferrous salt solution containing a relatively great quantity of ferric salt, it would be necessary in order to avoid precipitation of ferric hydroxide to utilize a pH at which a noticeable hydrogen discharge occurs. Therefore, in order to obtain good electrolysis results, one must limit the concentration of the ferric salt present in the solution in comparison with the ferrous salt concentration, at least near the cathode. In general, the ferric salt should not amount to more than of the weight of the ferrous salt. However, such a limitation is not absolute and in some cases it may be exceeded, although to the detriment of the electric output.

The influence of the cathode current density on the electrolysis is as follows. All other conditions remaining equal, increasing the cathode current density causes the following phenomena to occur successively; first, there is a discharge of hydrogen, then there is a compact deposit of metal having a smooth surface, then there is a compact metallic deposit with a somewhat granular surface, then there is a pulverulent strongly adherent deposit and finally there is a pulverulent deposit which comes off the cathode more and more easily. Although these successive occurrences are in general as described, owing to the presence of other factors, it is not possible to give exact limits to the ranges of current density at which each of these factors occurs.

However, all other conditions being equal, the current density at which an easily detachable powder is formed increases as the concentration of ferrous salt in the solution increases.

Although a Wide range of bath temperatures can be employed, high temperatures are generally avoided for the following reason. As the temperature of the bath increases, the electrical resistance of the bath decreases which requires that a lower pH be employed in order to avoid hydrolysis of the iron salts. This lowering of the pH involves the disadvantage of increasing the loss of current because of hydrogen cathodic discharge.

From the above it can be seen that by varying some factors it is possible to give to the others very different values.

Particularly favorable results are obtained by using ferrous salt concentrations between 0.05 and 0.5 mol per liter, ferric salt concentrations less than ,5 of the ferrous salt concentration, a pH between 3 and 5, a temperature which is low or near the ambient temperature and a cathode current density between 0.5 and 5 a./cm

Although most ferrous salts are utilizable in the process, chloride and sulphate are particularly suitable. We can also use certain ferric salts provided the resulting ferric ions can be reduced near the cathode, with the formation of ferrous ions.

The cathode must be constituted of a material which does not react with the bath, for example, graphite, iron, copper or any metal more electropositive than iron.

The anode may be constituted by an inert material which is not attacked by the bath as, for example, graphite or platinum. In this case, it is necessary to utilize an electrolyzer with a diaphragm to prevent the acids and other products formed near the anode from diffusing to- Ward the cathodic area. The anode may also be a soluble anode of iron and then it is possible to utilize an electrolyzer Without a diaphragm. But, in this case, the anode current density will have to be controlled as a function of the ratio of the ferric ions to ferrous ions desired to be kept in the electrolytic bath. Without our being able to explain the observed phenomena with certainty, it has been observed that the iron anode dissolves mostly in the state of ferrous ions under low anode current densities and mostly in the state of ferric ions under high anode current densities. Therefore, in the case of an electrolysis without a diaphragm and with a soluble iron anode, the anode current density should be higher the higher the ratio of ferric ions to ferrous ions desired in the solution.

The preparation of non-oxidized or only slightly oxidized iron powder is dependent upon the absence of oxygen, free or dissolved, in the bath. As already known, iron in a very finely divided state reacts rapidly with oxygen. It can ignite in the open air and oxidizes very rapidly in an aqueous solution in contact with air. The speed of oxidation of electrolytically formed pulverulent iron can reach its speed of formation. To protect the product against oxidation, the electrolysis may be carried out in a closed apparatus in which the air has been replaced by a gas carefully freed from oxygen, the apparatus being provided with a safety device to avoid an overpressure which might result from the possible discharge of hydrogen. An open apparatus can be employed in which the bath is covered by a layer of liquid such, for example, as gasoline, which is impremeable to air. A gas carefully freed from oxygen is bubbled through the bath. Argon and helium are particularly suitable gases for this purpose. Besides, all the various above-indicated means may be used at the same time.

Oxidized iron powder may be obtained if the oxidoreduction potential of the bath is too high. Said potential is connected with the ratio of ferric ions to ferrous ions in the bath. Generally, we employ an oxido-reduction potential under +650 milli-volts, the oxido-reduction potential being measured relative to the normal hydrogen electrode.

The iron powder formed on the cathode remains more or less adherent on it and must be detached. The deposit can be removed from the cathode by striking it with the help of suitable mechanically or electromagnetically controlled devices. It may also be submitted to intermittent brushing or scraping, the cathode being either fixed or movable. A suitable way of accomplishing such a scraping with a movable cathode consists in introducing the cathode into an insulating sheath having a section slightly larger than the initial section of the cathode. In this case, the current is practically interrupted when the cathode is inside its sheath and is restored when the cathode extends out of its sheath. The powder is removed from the cathode by moving the cathode inside its sheath periodically, thereby scraping ofi the powder from the cathode. The powder thus detached from the cathode remains in the bath. It may fall down to the bottom of the bath or, under the influence of the gas bubbled through the bath or of hydrogen it has absorbed, it may remain in suspension in the bath or may even collect in the upper part of the bath near the separation surface of the bath from the liquid or gaseous layer covering it.

The powder may be separated from the bath by filtration. A non-oxidized, uniform, very fine iron powder, contrary to an oxidized iron powder, does not clog up the usual filtering surfaces. But such a method of separating the iron powder from the bath requires that great operating precautions be taken to avoid all contact with oxygen. After filtering, the powder is carefully washed, first with water and then with acetone, both previously freed from oxygen, and the powder is then kept until its utilization away from all contact with oxygen. Liquids such as acetone are particularly suitable for such preservation.

A particularly suitable method of collecting and then removing the iron powder from the bath consists in submitting it in the electrolysis bath to the action of a magnetic field which causes it to gather at the bottom of the bath in a small apparent volume. Then it is washed by decanting, the successive washing liquids being deaerated water which may be slightly acidulated in order to remove traces of oxides and then deaerated acetone. The washing and decanting operations are performed in the presence of a magnetic field and in an atmosphere freed from oxygen.

The powders thus obtained are very fine. The elementary grains are so small as to be undiscernable by microscopic examination of the visible heaps of powder. The advantage of the process consists in the fact that powders are obtained having specific magnetizations and determined coercive forces according to the operating conditions.

The specific magnetization may reach that of pure iron, 217 U.E.M. c.g.s. The less oxide the powder contains the higher is its specific magnetization. Generally, the less ferric salts in the bath the higher is the specific magnetization.

The coercive force may vary considerably and depends upon several factors. It increases with the cathode current density. It is influenced by the quantity of ferric ions contained in the solution and generally is higher if the bath contains a small proportion of ferric salt. If the duration of the periods during which the powder is allowed to collect on the cathode without being detached is progressively increased, a rapid increase of coercive force is observed for each cathode current density. Then, from a certain value of said duration, the coercive force is only slightly influenced. This value of duration is all the smaller as the cathode current density is higher. The following table which gives the coercive forces in oersteds shows that, for relatively small current densities of 1 to 1.5 a./cm. high coercive forces are not obtained if the periods during which the powder is allowed to grow on the cathode are of one second duration and that, on the contrary, if the current density is 2.5 a./cm. high and stable values of coercive force are obtained if the periods during which the powder is allowed to collect on the anode are of the order of one second. The indicated current densities refer to the initial surface of the cathode. The active cathode surface increases between two scraping operations and, consequently, the elfecti ve current density decreases.

TABLE I Coercive Force Values Duration of the Periods in Sec.

In order to obtain reproducible results, it is advisable to use an arrangement in which the electric current between the anode and cathode is distributed as uniformly as possible. A particularly suitable arrangement is to use a cylindrical anode and a coaxial cylindrical central cathode.

The following examples further illustrate our invention.

EXAMPLE 1 We electrolyzed a solution containing 0.3 mol per liter of ferrous chloride and a small quantity of ferric chloride obtained by electrolytic reduction of iron perchloride and characterized by a redox potential of +470 millivolts compared to the normal hydrogen electrode and a pH of 3.4. The cathode was a copper cylinder of 4 mm. in diameter, the anode a cylinder of sheet steel of 200 mm.

in diameter. The cathode current density was 1 a./cm.

the anode current density 0.01 a./cm. and the bath temperature 15 C.

The bath was covered by a gasoline layer and a slight current of argon was bubbled through the bath. The 1 intervals by striking the top of the cathode. The powder EXAMPLE 2 The apparatus employed was identical with that of Example 1. The electrolysis solution was obtained by electrolytic reduction of iron perchloride and contained 0.1 mol per liter of ferrous chloride and a small quantity of ferric chloride. The initial pH was 3.7, the cathode current density 1.6 a./cm. the anode current density 0.032 a./cm. the temperature 17 C. The bath was also covered by gasoline and a slight bubbling of argon was maintained in it during the electrolysis. The powder was detached every 3 seconds by shocks on the cathode top and then separated by filtration.

The obtained powder had a coercive force of 750 oersteds and a specific magnetization of 181 U.E.M. c.g.s.

EXAMPLE 3 The apparatus employed was identical to that used in Example 1. The electrolysis bath was a solution of ferrous chloride practically free from ferric chloride and was obtained by dissolving pure salt in unoxidized distilled water. The concentration of the solution was 0.2 mol per liter, the cathode current density 1 a./cm. the anode current density 0.04 a./cm. the pH 3.0, the temperature 15 C. The bath was covered by gasoline and a slight bubbling of argon was maintained in it. The powder was detached by shocks on the cathode top every 3 seconds and separated by filtration.

The obtained powder had a coercive force of 256 oersteds and a specific magnetization of 217 U.E.M. c.g.s.

EXAMPLE '4- The apparatus comprised a cylindrical anode of 60 mm. diameter formed by an iron sheet and a cathode formed by a copper wire of 1.1 mm. diameter and arranged concentrically within the anode. The cathode was movable into and out of a glass sheath whose internal diameter was slightly greater than 1.1 mm. and Was mechanically connected with the movable part of an electromagnet. Thus, the cathode could be made to enter the glass sheath at will in order to scrape off from the cathode the powder which had been deposited on it.

The electrolysis solution contained 0.1 mol per liter of ferrous chloride, practically no ferric salt and its pH was 3. However, the normal p-H after half-an-hours electrolysis was 4.8, which value was maintained constant during several hours.

The cathode current density was 2.5 a./cm. the anode current density 0.005 a./cm. and the temperature of the bath 25 C. The bath was covered by gasoline and a slight current of argon was bubbled through it.

The cathode was periodically drawn into its sheath at intervals of 2.5 seconds in order to detach the iron powder from the cathode.

The powder was separated from the bath and washed by decantation in a magnetic field in an atmosphere freed from oxygen. It had a coercive force of 460 oersteds and a specific magnetization of 172 U.E.M. c.g.s.

The invention is not limited to the preferred embodiment but may be otherwise embodied or practiced within the scope of the following claims.

We claim:

1. A process of preparing iron powder having a high coercive force, which comprises providing a ferrous salt solution having a pH between 3 and 5 and containing 0.05 to 0.5 mol per liter of ferrous salt and not over /1 as much ferric salt by weight as ferrous salt, maintaining said solution out of contact with oxygen and passing an electric current from an anode to a cathode through said solution at a cathode current density between 0.5 and 5 amperes per square centimeter to deposit iron powder on the cathode, periodically detaching from the cathode the iron powder deposited thereon, and separating the iron powder from the solution while maintaining it out of contact with oxygen.

2. A process according to claim 1, wherein the ferrous salt is ferrous chloride and the ferric salt is ferric chloride.

3. A process of preparing iron powder having a high coercive force, which comprises providing a ferrous salt solution having a pH between 3 and 5 and containing 0.05 to 0.5 mol per liter of ferrous salt and not over as much ferric salt by weight as ferrous salt, covering said solution with a liquid which is impermeable to air, bubbling a non-oxidizing gas through said solution and passing an electric current from an anode to a cathode through said solution at a cathode current density between 0.5 and 5 amperes per square centimeter to deposit iron powder on the cathode, periodically detaching from the cathode the iron powder deposited thereon, and separating the iron powder from the solution while maintaining it out of cont act with oxygen.

4. A process of preparing iron powder having a high coercive force, which comprises providing a ferrous salt solution having a pH between 3 and 5 and containing about 0.1 to 0.3 mol per liter of ferrous salt and not over as much ferric salt by weight as ferrous salt, maintaining said solution out of contact with oxygen and passing an electric current from an anode to a cathode through said solution at a cathode current density between 1.0 and 2.5 amperes per square centimeter to deposit iron powder on the cathode, periodically detaching from the cathode the iron powder deposited. thereon, and separating the iron powder from the solution while maintaining it out of contact with oxygen.

5. A process according to claim 4, wherein the ferrous salt solution contains about 0.1 mol per liter of ferrous salt.

References Cited in the file of this patent UNITED STATES PATENTS 411,042 Kolle Sept. 11, 1889 501,996 Emmens July 25, 1893 1,254,056 Moore Jan. 22, 1918 1,804,924 Fitzpatrick et a1. May 12, 1931 1,817,527 Schlotter Aug. 4, 1931 2,287,082 Bauer June 23, 1942 2,464,168 Balke Mar. 8, 1949 2,481,079 Casey Sept. 6, 1949 2,786,809 Raynes Mar. 26, 1957 OTHER REFERENCES Metal Treatment and Drop Forging (Summer 1950), pp. 118-126. 

1. A PROCESS OF PREPARING IRON POWDER HAVING A HIGH COERCIVE FORCE, WHICH COMPRISES PROVIDING A FERROUS SALT SOLUTION HAVING A PH BETWEEN 3 TO 5 AND CONTAINING 0.05 TO 0.5 MOL PER LITER OF FERROUS SALT AND NOT OVER 1/100 AS MUCH FERRIC SALT BY WEIGHT AS FERROUS SALT, MAINTAINING SAID SOLUTION OUT OF CONTACT WITH OXYGEN AND PASSING AN ELECTRIC CURRENT FROM AN ANODE TO A CATHODE THROUGH SAID SOLUTION AT A CATHODE CURRENT DENSITY BETWEEN 0.5 AND 5 AMPERS PER SQUARE CENTIMETER TO DEPOSIT IRON POWDER ON THE CATHODE, PERIODICALLY DETACHING FROM THE CATHODE THE IRON POWDER DEPOSITED THEREON, AND SEPARATING THE IRON POWDER FROM THE SOLUTION WHILE MAINTAINING IT OUT OF CONTACT WITH OXYGEN. 